Low-pressure chemical vapor deposition (LPCVD) reactors are widely used in the manufacture of thin-film devices such as those employed in microelectronic circuitry. The manufacture of such devices frequently depends critically on the thicknesses, compositions, and physical properties of the various thin-film materials. To achieve the needed control of thin film chemical and physical properties, the LPCVD processes by which these multilayer devices are manufactured need to be performed under precise control. Further, in order to minimize waste in industrial LPCVD manufacturing processes, unpredictable run-to-run variations need to be minimized or eliminated. This requires that the LPCVD processes be performed with strict control of the key operating parameters, including, in addition to reactor temperature and pressure, the rates at which the vaporized chemical precursors, oxidants, inert diluents, and other gaseous components are delivered to the chemical reactor.
Precise control of the delivery rates of gaseous oxidants or of inert diluents may be readily achieved by the use of mass flow controllers which are widely available from commercial sources. Also, there are commercially available flow controllers that are designed to deliver various vaporized chemical precursors at controlled rates to the inlet ports of LPCVD reactors. Generally, such chemical precursors are liquids at normal temperatures and pressures. Two distinctly different operations must be carried out to provide for a controlled vapor delivery rate: vaporization and flow control. The order of the operations is not fundamentally important. Hence, a gas flow controller may be employed downstream from a liquid vaporizer, or a vaporizer may be employed downstream of a liquid flow controller. The approach used with a particular precursor liquid is based on consideration of the chemical and physical properties of the material including the vapor pressure curve, the thermal stability of the material, and sensitivity to the presence of impurities.
Cavitation (formation of gas or vapor filled cavities within liquids by mechanical forces) at low mass flow in the liquid feed lines may result in a poorly controlled mass flow. In order to prevent cavitation of the liquid in the liquid feed lines, a flow restriction is often used to provide sufficient back pressure in the liquid feed lines. A shut-off valve is usually separate from and upstream of the restrictor. A purge line and purge valve are usually installed downstream of the flow restrictor to allow purging of the system. This configuration presents several disadvantages. First, for small liquid flows the restrictor must be a very narrow (typically from 0.005" to 0.040" inner diameter) and long (several inches) tube, which is extremely susceptible to clogging when water sensitive or hydrocarbon reactive liquids are used. Second, the pressure drop over the restrictor is a function of mass flow, which means individual restrictor optimization is required for each different mass flow. Third, the volume of liquid in the shut-off valve, the restrictor, and in the tubing/fittings between these components can still evaporate after closure of the shut-off valve, dramatically increasing the response time of the liquid delivery system. Fourth, cavitation may occur in the restrictor or shut-off valve during transients when the flow is very low which results in poor repeatability of the mass flow in non steady state situations such as precursor shut-off during wafer transfer in a single wafer LPCVD reactor. Finally, the separate placement of a purge valve generally results in a dead zone resulting in inadequate purging in a portion of the liquid line.